Phase diagram of tyioxapol and water — I

Abstract

Tyloxapol, a nonionic liquid polymer of the alkyi aryl polyether alcohol type, described as freely soluble in water, is widely used in pharmacy. At room temperature and above, tyloxapol does not dissolve immediately in water but forms liquid crystalline structures in the contact zone with water resulting among others in the slow formation of aqueous one-phase systems. Macroscopically isotropic aqueous systems with a concentration-dependent viscosity are formed below 35 wt% tyloxapol. Around 35 wt% there is a narrow two-phase coexistence region. From 37.5 to 65 wt% textures of lyotropic liquid crystalline hexagonal phases are observed in the polarizing microscope. Solubilization tests indicate a hexagonal phase. This phase exists in a broad concentration and temperature range as derived from hot-stage polarizing microscopy and differential scanning calorimetry. The hexagonal phase exhibits the highest transition temperature (59°C) and the highest transition heats (0.5 J/g) around 50 wt%. Transmission electron microscopy (TEM) observations indicate a supramolecular arrangement consisting of parallel cylinders of non-uniform length resulting in two-dimensional order. In a narrow concentration range around 70 wt% and below 30°C, there is a stiff lyotropic liquid crystalline cubic phase with a transition heat of 1.3 J/g. Between 74 and 80 wt% at room temperature there is a liquid crystalline lamellar phase with low viscosity exhibiting no measurable transition heats. TEM observations indicate a supramolecular arrangement consisting of extended stacked lamellae exhibiting only one-dimensional order. The retransformation of the cubic phase from the melt is highly retarded. In contrast, the liquid crystalline hexagonal phase and lamellar phase are immediately formed on cooling of the melts to room temperature. Above 80 wt% the systems represent crystal dispersions.